Thermal energy, such as generated by high intensity focused ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers driven by electric signals to produce ultrasonic energy have been suggested that may be placed external to the patient but in close proximity to the tissue to be ablated. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a “focal zone” corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is coagulated. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of “sonications” is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
In such focused ultrasound systems, the transducer is preferably geometrically shaped and positioned so that the ultrasonic energy is focused at a “focal zone” corresponding to the target tissue region, heating the region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. For example, this series of “sonications” may be used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
By way of illustration, FIG. 1A depicts a phased array transducer 10 having a “spherical cap” shape. The transducer 10 includes a plurality of concentric rings 12 disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings 12 generally have equal surface areas and may also be divided circumferentially 14 into a plurality of curved transducer sectors, or elements 16, creating a “tiling” of the face of the transducer 10. The transducer elements 16 are constructed of a piezoelectric material such that, upon being driven with a sinus wave near the resonant frequency of the piezoelectric material, the elements 16 vibrate according to the phase and amplitude of the exciting sinus wave, thereby creating the desired ultrasonic wave energy.
As illustrated in FIG. 1B, the phase shift and amplitude of the respective sinus “drive signal” for each transducer element 16 is individually controlled so as to sum the emitted ultrasonic wave energy 18 at a focal zone 20 having a desired mode of focused planar and volumetric pattern. This is accomplished by coordinating the signal phase of the respective transducer elements 16 in such a manner that they constructively interfere at specific locations, and destructively cancel at other locations. For example, if each of the elements 16 are driven with drive signals that are in phase with one another, (known as “mode 0”), the emitted ultrasonic wave energy 18 are focused at a relatively narrow focal zone. Alternatively, the elements 16 may be driven with respective drive signals that are in a predetermined shifted-phase relationship with one another (referred to in U.S. Pat. No. 4,865,042 to Umemura et al. as “mode n”). This results in a focal zone that includes a plurality of 2n zones disposed about an annulus, i.e., generally defining an annular shape, creating a wider focus that causes necrosis of a larger tissue region within a focal plane intersecting the focal zone. Multiple shapes of the focal spot can be created by controlling the relative phases and amplitudes of the emmitted energy from the array, including steering and scanning of the beam, enabling electronic control of the focused beam to cover and treat multiple of spots in the defined zone of a defined tumor inside the body.
More advanced techniques for obtaining specific focal distances and shapes are disclosed in U.S. patent application Ser. No. 09/626,176, filed Jul. 27, 2000, entitled “Systems and Methods for Controlling Distribution of Acoustic Energy Around a Focal Point Using a Focused Ultrasound System;” U.S. patent application Ser. No. 09/556,095, filed Apr. 21, 2000, entitled “Systems and Methods for Reducing Secondary Hot Spots in a Phased Array Focused Ultrasound System;” and U.S. patent application Ser. No. 09/557,078, filed Apr. 21, 2000, entitled “Systems and Methods for Creating Longer Necrosed Volumes Using a Phased Array Focused Ultrasound System.” The foregoing (commonly assigned) patent applications, along with U.S. Pat. No. 4,865,042, are all hereby incorporated by reference for all they teach and disclose.
It is significant to implementing these focal positioning and shaping techniques to provide a transducer control system that allows the phase of each transducer element to be independently controlled. To provide for precise positioning and dynamic movement and reshaping of the focal zone, it is desirable to be able to alter the phase and/or amplitude of the individual elements relatively fast, e.g., in the μ second range, to allow switching between focal points or modes of operation. As taught in the above-incorporated U.S. patent application Ser. No. 09/556,095, it is also desirable to be able to rapidly change the drive signal frequency of one or more elements.
Further, in a MRI-guided focused ultrasound system, it is desirable to be able to drive the ultrasound transducer array without creating electrical harmonics, noise, or fields that interfere with the ultra-sensitive receiver signals that create the images. A system for individually controlling and dynamically changing the phase and amplitude of each transducer element drive signal in phased array focused ultrasound transducer in a manner which does not interfere with the imaging system is taught in commonly assigned U.S. patent application Ser. No. [not-yet-assigned; Lyon & Lyon Attorney Docket No. 254/189, entitled “Systems and Methods for Controlling a Phased Array Focussed Ultrasound System,”], which was filed on the same date herewith and which is hereby incorporated by reference for all it teaches and discloses.
Notably, after the delivery of a thermal dose, e.g., ultrasound sonication, a cooling period is required to avoid harmful and painful heat build up in healthy tissue adjacent a target tissue structure. This cooling period may be significantly longer than the thermal dosing period. Since a large number of sonications may be required in order to fully ablate the target tissue site, the overall time required can be significant. If the procedure is MRI-guided, this means that the patient must remain motionless in a MRI machine for a significant period of time, which can be very stressful. At the same time, it may be critical that the entire target tissue structure be ablated (such as, e.g., in the case of a malignant cancer tumor), and that the procedure not take any short cuts just in the name of patient comfort.
Accordingly, it would be desirable to provide systems and methods for treating a tissue region using thermal energy, such as focused ultrasound energy, wherein the thermal dosing is applied in a more efficient and effective manner.